RE: Index Skip Scan (new UniqueKeys)

Hi Dmitry,

Also took another look at the patch now, and found a case of incorrect data. It looks related to the new way of creating the paths, as I can't recall seeing this in earlier versions.

create table t1 as select a,b,b%5 as c, random() as d from generate_series(1, 10) a, generate_series(1,100) b;
create index on t1 (a,b,c);

postgres=# explain select distinct on (a) * from t1 order by a,b desc,c;
QUERY PLAN
-------------------------------------------------------------------------------
Sort (cost=2.92..2.94 rows=10 width=20)
Sort Key: a, b DESC, c
-> Index Scan using t1_a_b_c_idx on t1 (cost=0.28..2.75 rows=10 width=20)
Skip scan: true
(4 rows)

With the 'order by a, b desc, c' we expect the value of column 'b' to always be 100. With index_skipscan enabled, it always gives 1 though. It's not correct that the planner chooses a skip scan followed by sort in this case.

-Floris